The invention relates to a circuit arrangement for reading out a sensor sensitive to light or X-rays comprising a radiation-sensitive element, more particularly a photodiode, whose charge can be read out by means of a switching diode, which for this purpose is put in its conductive state by means of a voltage temporarily applied to it.
In such circuit arrangements for reading out sensors sensitive to light or X-rays, there is the problem that, after the voltage has been switched on, the switching diode becomes not sufficiently conducting especially if the capacitance of the photodiode was charged by the sensor element to an only comparatively small extent. A realizable read-out of the charge stored in the capacitance of the photodiode is therefore possible only with comparatively large quantities of charge. Especially for the field of use of X-ray imaging, however, the quantities of charge are comparatively small so that, for example, in this field of use such a circuit arrangement cannot be employed.
Further, it is known, for example, from DE-OS 3531448 which corresponds to U.S. Pat. No. 4,800,557 to use for reading out such radiation-sensitive sensors instead of a switching diode field effect transistors in thin-film technology. However, such field effect transistors produce on account of parasitic capacitances a disturbing offset during reading out. Further, driving the gate of each field effect transistor requires a further lead so that the yield in production is reduced.